1. Field of the Invention
This invention relates generally to fuel reformers, and, more particularly, to level control in a water tank of a fuel reformer.
2. Description of the Related Art
Fuel cells are beginning to replace conventional sources of power in a variety of contexts, including cars, buses, houses, commercial buildings, and the like. The three main arguments in favor of fuel cells are abundance, efficiency, and cleanliness. First, the primary fuel for fuel cells is hydrogen, which is the most abundant element in the universe. Second, the efficiency of a fuel cell may exceed the Carnot Cycle limit while operating at a relatively low temperature. For example, a fuel cell operating at 80° C. is typically two to three times as efficient as an internal combustion engine, which may also require substantially higher operating temperatures. Third, the by-products of fuel cell operation are typically benign. For example, de-ionized water and fluorine are the only by-products of a fuel cell powered entirely by hydrogen.
One type of fuel cell is a polymer electrolyte membrane (PEM) fuel cell. This type of fuel cell may also be referred to as a proton exchange membrane fuel cell, a solid polymer electrolyte (SPE) fuel cell, or a polymer electrolyte fuel cell. In operation, hydrogen and oxygen are introduced into an anode and a cathode, respectively, of the PEM fuel cell. The hydrogen dissociates into electrons and protons, and the protons diffuse through an electrolyte membrane, such as the Nafion™ membrane produced by DuPont, that separates the anode from the cathode. When the protons reach the cathode, they react with the oxygen to form water and heat. The electrons are collected on the anode side of a bipolar collector plate and an opposite charge is collected on the cathode side of the bipolar plate. The charge difference results in an electric potential, which generates a voltage difference of approximately 0.7 volts between the anode and the cathode.
There are, however, a number of drawbacks to using pure hydrogen as the primary fuel for a fuel cell. Hydrogen gas has a relatively low energy density and there is as yet no infrastructure for hydrogen gas distribution. Although the energy density may be increased by liquefying the hydrogen, liquefying adds to the overall cost of the hydrogen and increases the energy required to use the hydrogen gas as a fuel. Furthermore, liquid hydrogen must be maintained at a low temperature and requires constant purging, and is therefore substantially more expensive to distribute than hydrogen gas. Thus, a number of primary fuels have been proposed, including natural gas, gasoline, propane, methanol, ethanol, naphtha, and the like. The proposed primary fuels either already have a distribution infrastructure in place or they are easier to handle and/or produce.
When using a primary fuel, a reformer, also referred to as a fuel processor, is used to produce hydrogen from the primary fuel. Three conventional reformer designs are steam reformers, partial oxidation reformers, and auto-thermal reformers. Steam reformers combine the primary fuel with steam and heat to produce a reformate containing a large percentage of hydrogen. The steam reforming reaction is endothermic and the heat required to operate the system is obtained by burning the primary fuel or excess reformate from an outlet of the fuel cell. Partial oxidation reformers combine the primary fuel with oxygen to produce a reformate containing a large percentage of hydrogen and carbon monoxide. The carbon monoxide then reacts with steam to increase the percentage of hydrogen in the reformate. Partial oxidation is an exothermic reaction that releases heat and, if integrated properly, the heat may be captured and used elsewhere in the system. Auto-thermal reformers combine the primary fuel with both steam and oxygen to achieve a heat balance wherein the exothermic partial oxidation reaction provides heat for the endothermic steam reforming reaction.
The water used to produce the steam in the reformer is typically drawn from a water tank. The water tank may have multiple input sources, such as condensate returned from the fuel cell, cooling loop return, fill lines from external utility connections, system surge caused by thermal expansion, and the like. Water ingress through the multiple input sources may raise the water level in the water tank in excess of a desirable level. Excess water in the water tank may cause the reformer and/or the fuel cell to operate in an undesirable manner or to fail altogether. For example, excess water may fill the reformer and/or the fuel cell or may disrupt the flow of gas through a section of the reactor.
The water tank typically also has multiple outputs, including the cooling loops, water provided to the fuel cell, water provided to the reformer, and the like. Egress through the multiple outputs may reduce the water to an undesirably low level, or remove the water entirely from the water tank. Low-water conditions in the water tank may cause the reformer and/or the fuel cell to operate in an undesirable manner or to fail altogether. For example, low water levels may reduce or stop the flow of process water to the reformer, which may reduce or stop the production of hydrogen fuel. For another example, low water levels may reduce or stop the flow of water to the fuel cell, which may reduce the water content of the electrolyte membrane and decrease the proton transfer in the PEM fuel cell.